A Linear Relationship between the Latitude Gradient and 26 Day Recurrent Variation in the Fluxes of Galactic Cosmic Rays and Anomalous Nuclear Components. I. Observations

Abstract

We find that there exists a linear relationship between the magnitude of latitude gradient and the amplitude of 26 day recurrent variations in the fluxes of galactic cosmic rays and anomalous nuclear components. This result is based on energetic charged particle measurements from the University of Chicago High Energy Telescope (HET) in the Cosmic Ray and Solar Particle Investigation (COSPIN) consortium on the Ulysses spacecraft during its mission to the solar poles and on similar measurements from IMP-8 spacecraft orbiting around Earth at 1 AU in the solar ecliptic. The linear relationship holds for recurrent cosmic-ray flux variations observed in the inner heliosphere at all latitudes including the high-latitude regions covered by Ulysses and in the equatorial by IMP-8, and it is roughly independent of particle energy and species. The relationship means that particles with large-latitude flux gradients are also strongly modulated by recurrent solar wind structures such as corotating interaction regions that appear in low- and middle-latitude regions. In the previous solar cycle, when the solar magnetic field had an opposite polarity, cosmic-ray measurements from the Voyager spacecraft and IMP-8 also show a linear relationship between the latitude gradient and the amplitude of recurrent variations even though the latitude gradient had a negative sign. With these observational facts, the three-dimensional flux distributions of cosmic rays in the inner heliosphere can be more easily measured and better understood because the linear relationship implies that there are similarities among different kinds of charged particles in their modulated flux distributions. These observations suggest that there is a common dominant modulation mechanism controlling both the global latitudinal distribution and the short-term temporal variation of cosmic-ray fluxes. A theoretic model to account for these observations and to understand cosmic-ray modulation in the three-dimensional heliosphere will be presented in Paper II.